This invention relates to multi-speed transmissions, in particular, transmissions having drive engaging devices energized by fluid pressure, and more particularly to the fluid circuits for energizing such drive engaging devices in response to the operator's selection of a desired speed and directional drive engaging device.
Certain forms of vehicle transmissions utilize clutches or brakes to activate each selected drive ratio for the selected drive direction. To avoid shocks and reduce stress and wear the control system is arranged to apply fluid pressure to the selected directional drive engaging device or selected speed drive engaging device in a gradual or modulated manner. Normally, one of the drive engaging devices, usually the directional device, is made larger and more wear resistant. The control system is designed to attain operating pressure in the larger and more wear resistant engaging device subsequent to reaching operating pressure in the smaller device. This insures that the higher capacity device is the one which sustains the shock of reestablishing torque flow from the engine through the transmission to the drive shaft.
In order to reduce shocks to the transmission, and primarily for safety reasons, the control systems are usually designed to provide still further functions. These further functions include automatic blocking of pressure from the directional drive engaging device in the event the engine is started up while the shift lever is at a position other than neutral. Furthermore, it is desirable to provide for automatic neutralization of the transmission when the vehicle brakes are applied, thereby reducing wear on the transmission.
Prior transmission control systems of the general type under discussion are disclosed, for example, in U.S. Pat. Nos. 3,091,976; 3,181,394; 3,198,027; 3,217,726; 3,386,540; 3,444,762; 3,468,194; 3,481,435 and 3,596,536. U.S. Pat. application Ser. No. 373,246, now Pat. No. 3,863,523 assigned to the assignee of this invention, also shows a hydraulic control system of the type under discussion in this application. In order to realize the system functions discussed above, it has been necessary to complicate these systems with separable valving components for such purposes as sensing when the transmission control lever is in neutral, forcing neutralized condition of the transmission in response to fluid signals from other vehicle components such as the brakes and for maintaining fluid pressure while the system is so neutralized.
U.S. Pat. No. 3,709,065 shows a simplified version of this type of transmission control system. U.S. Pat. application Ser. No. 421,065, now U.S. Pat. No. 3,882,738 assigned to the assignee of this invention is an improvement on the system of U.S. Pat. No. 3,709,065 wherein the check valve function is included in the function of the safety differential valve spool. These previous patents and applications have suffered the disability of not providing sufficient fluid flow in large volume clutches thereby increasing torque interruptions during speed changes. Further, in this prior art, fluid flow in the differential check and safety valve has been restricted by complex porting systems in valve spools, such porting systems increasing the cost of manufacturing by requiring difficult machining processes either on the valve spool or in the valve housing itself. Simplification was possible in the differential check and safety valve by extending the length of the spool and housing but such lengthening was not considered desirable.